Errorless protection switching systems, e.g., for digital radio, are known. In these systems, signals carried on a service channel are switched to be carried on an alternate channel upon the detection of a predetermined threshold number of errors at the receiver. To accomplish such a switch, these prior switching systems must compensate for the different transmission delays between the service channel and the alternate channel. In such radio systems, delays are minimized because the service channel and alternate channel traverse the same physical transmission route. Therefore, the differential delay, i.e., the difference in transmission delay between the service channel and the alternate channel, is minimal. Compensation for the differential delay is obtained by employing on each service channel a delay build-out equal to the maximum differential delay, .DELTA.. The delay of the service channel combined with the build-out delay is always greater than the delay inherent in the alternate channel. Furthermore, a variable delay, the length of which ranges from zero to two times the maximum differential delay, can be controllably added to the alternate channel. As a result, a delay can always be added to the alternate channel so that the total delay of the alternate channel is equal to the total delay of the service channel. Therefore, although the alternate channel without build-out delay can be shorter or longer than any given service channel without build-out delay, the total delay of each channel with their respective build-out delays can be equalized. Once delay equality of the channels has been achieved, data may be errorlessly switched from the service channel to the alternate channel.
Such an errorless switching system can only be utilized when the bit stream transported on the alternate channel is identical to the bit stream transported on the service channel. However, when employing a signal format, for example the SONET format, in which the starting location of a payload, i.e., a predetermined unit of data, can float within each frame, the bit streams arriving at the receiver from the service channel and the alternate channel may not be identical. Additionally, the overhead bytes of the signal carried on the alternate channel may be different than the overhead bytes of the signal carried on the service channel, even if the starting location of the payload in corresponding frames is the same. For example, signals originating from a SONET network may need to be rerouted from the service channel to an alternate channel which passes through an intermediate central office. To keep the payload synchronous with the SONET network the payload may be shifted within the frame. In addition, the overhead bytes themselves can be different because some of them may be utilized for auxiliary functions between two network elements, e.g., between the originating element and the added intermediate element through which the signal is rerouted. This would result in the bit streams comprising the signals on the service and alternate channels arriving at the receiving end being different. In such situations, prior errorless switching systems and techniques cannot achieve an errorless switch.